A characteristic feature of periodontitis is the existence of a variety of volatile sulfur compounds produced at sites of destructive disease. Hydrogen sulfide (H2S) is a major component of this family, which is a primary factor in halitosis related to periodontitis, and importantly, can affect host molecules and cells. Previous studies have identified a limited repertoire of oral microorganisms that can produce and survive in microenvironments with large amounts of H2S; however, the metabolic enzymes involved in the production of H2S are poorly understood. Substantial literature has identified T. denticola as one of the few oral bacteria with the capacity to produce and grow in high levels of H2S. The Principal Investigator has previously isolated and characterized an enzyme, cystalysin. This enzyme is unique to T. denticola and has a substrate specificity for L-cysteine, yielding pyruvate, NH3, and importantly, H2S. This proposal develops the general hypothesis that H2S is a significant effector of host responses in the local microenvironment of the periodontal disease site and, furthermore, that H2S-forming enzymes should be considered virulence determinants for T. denticola. Three specific aims are developed using biochemical, molecular genetic, and cell biologic studies to address this hypothesis: Specific Aim 1: To construct isogenic mutants altered in cystalysin-directed H2S producing capacity; Specific Aim 2: To biochemically characterize the enzyme pathway(s) involved in H2S production from glutathione; and, Specific Aim 3: To determine the effects of an H2S environment created by T. denticola on host inflammatory/immune functions. T. denticola has been identified as an important member of a specific consortia of microorganisms considered as etiologic in the development and progression of destruction of the periodontium. This proposal is designed to provide critical information related to the production of H2S by T. denticola, and for the first time to delineate the potential functions of H2S, which could deleteriously affect the homeostasis, maintained by molecules and cells in the periodontium. The outcomes would also be expected to provide an impetus to further target the enzymatic pathways contributing to H2S production as an innovative intervention strategy.